The Battle Master fighter has the Trip Attack maneuver (PHB, p. 74):
When you hit a creature with a weapon attack, you can expend one superiority die to attempt to knock the target down. You add the superiority die to the attack’s damage roll, and if the target is Large or smaller, it must make a Strength saving throw. On a failed save, you knock the target prone.
By R.A.W., you wait to see if the attack hits before rolling the superiority die for Trip Attack.
Is there a rule which demands that the DM resolve the damage and determine whether the target is dead, before a player ends up wasting their superiority dice on a pointless maneuver?